The invention relates to a method of producing an optically transparent substrate and to a method of producing a lighte-mitting semiconductor chip with an optically transparent substrate.
Transparent substrates are of great importance, in particular for LED (Light-Emitting Diode) chips, because their light-producing structures generally emit light in all geometrical directions, and consequently also toward the substrate. To achieve the highest luminous efficiency, it is therefore desirable to extract even that portion of the light from the chip that is emitted toward the substrate.
In particular in the case of LED chips with light-producing LED structures based on (AlGa)InP or AlGaAs, this entails the problem that, for these systems of materials, the only lattice-matched substrate which can currently be produced with economically acceptable technical outlayxe2x80x94essentially a GaAs substratexe2x80x94is optically absorbing at the wavelengths produced by the LED structures in question. The band gap of GaAs is less than the emission energy of the LED structures.
The result of leaving the GaAs substrate in the finished LED chip is consequently that a large proportion (up to 50%) of the radiation emitted by the LED structure is already lost in the chip due to absorption in the substrate.
One possibility of avoiding this problem is disclosed in U.S. Pat. No. 5,376,580. In the method described therein, in order to produce an LED chip with a light-emitting LED structure based on AlGaInP or AlGaAs, an AlGaInP or AlGaAs LED structure, respectively, is first epitaxially grown on a GaAs growth substrate.
The LED structure is subsequently separated from the absorbing GaAs substrate and bonded through the use of wafer bonding to a substrate, e.g. a GaP substrate, which has a high electrical conductivity and is optically transparent for the radiation in question.
The absorbing GaAs substrate is removed before or after the wafer bonding. Removal after the wafer bonding has advantages, in terms of the risk of damage to the LED structure during the removal of the absorbing GaAs substrate or during the wafer bonding, in comparison with removal before the wafer bonding.
In the above-described method, however, the epitaxially grown LED structure, whose electrical and optical properties are known to depend to a considerable extent on the crystal quality of the epitaxial layers, is subjected to additional mechanical (e.g. grinding, polishing etc.) and/or chemical method steps (e.g. etching) when separating the absorbing GaAs substrate, which may damage the epitaxially grown LED structure.
The availability of transparent substrates is very important not only in the case of the system of materials described above but also, for example, in the case of light-emitting semiconductor chips having other systems of materials, such as GaN and InGaN.
It is accordingly an object of the invention to provide a method of producing an optically transparent substrate which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and onto which a semiconductor structure can be epitaxially grown. A further object of the invention is to provide a method of producing a light-emitting semiconductor chip with an optically transparent substrate. With the foregoing and other objects in view there is provided, in accordance with the invention, a method of producing an optically transparent substrate, the method includes the steps of:
providing a lattice-matched substrate;
epitaxially growing a substrate layer on the lattice-matched substrate;
connecting the substrate layer to an optically transparent layer at a side of the substrate layer facing away from the lattice-matched substrate by using a wafer-bonding process; and
removing the lattice-matched substrate from a composite formed of the substrate layer and the optically transparent layer.
In other words, the first-mentioned object of the invention is achieved by the following method steps:
a) epitaxial growth of a substrate layer on a lattice-matched substrate, which is optimized in terms of the parameters for the epitaxial growth of the substrate layer and, for example, is optically absorbing and/or electrically insulating,
b) application of a prefabricated optically transparent layer onto the side of the substrate layer facing away from the lattice-matched substrate, by using a wafer bonding process, and
c) removal of the lattice-matched substrate from the composite formed of the substrate layer and the optically transparent layer.
The method according to the invention has the advantage that the epitaxially grown semiconductor structure can be substantially spared from mechanical and/or chemical stresses.
According to another mode of the invention, the substrate layer is provided as an optically transparent substrate layer.
Using the method according to the invention, it is advantageously possible to produce a substrate in which, on the one hand, the substrate layer allows an epitaxial growth of a semiconductor structure and, on the other hand, the material of the transparent layer is configured principally for an optical transparency which is optimized with respect to the intended use of the substrate.
The lattice constant of the material of the bonded optically transparent layer does not play any role in this regard. Only the substrate layer, which is typically much thinner than the transparent layer and which consequently plays only a subordinate role in terms of transparency and electrical conductivity, is used as a xe2x80x9csubstratexe2x80x9d for the epitaxial growth e.g. of a light-emitting layer sequence, and must consequently be lattice-matched thereto.
Preferably, the substrate layer is already formed of a material which is optically transparent for the radiation emitted by, for example, a light-emitting semiconductor structure which is provided. Light losses due to absorption in the substrate are thereby advantageously reduced even further.
In order to remove the optically absorbing lattice-matched substrate, it is possible to utilize methods which are known in this context, such as mechanical grinding, lapping, selective wet chemical etching, reactive ion etching or combinations of these methods (cf. e.g. U.S. Pat. No. 5,376,580). A method for the nondestructive removal of the lattice-matched substrate, which permits re-use of the substrate and which may also be utilized in the method according to the invention, is for example described in U.S. Pat. No. 5,877,070.
In terms of suitable methods for bonding the substrate layer to the optically transparent substrate it is likewise possible, for example, to employ the methods of so-called wafer bonding known from the prior art. Various suitable methods are described, e.g. again in U.S. Pat. No. 5,376,580.
The method is suitable, in particular, for the production of light-emitting diode chips with active light-emitting diode structures, for which no optically transparent lattice-matched substrate material is available or for which a lattice-matched transparent substrate cannot be produced with economically acceptable technical outlay.
With the objects of the invention in view there is also provided, a method of producing a light-emitting semiconductor chip structure with an optically transparent substrate, the method includes the steps of:
providing a lattice-matched substrate;
epitaxially growing a substrate layer on the lattice-matched substrate;
connecting the substrate layer to an optically transparent layer at a side of the substrate layer facing away from the lattice-matched substrate by using a wafer-bonding process;
removing the lattice-matched substrate from a composite formed of the substrate layer and the optically transparent layer; and
epitaxially growing a light-emitting semiconductor structure onto the substrate layer.
In other words, the second-mentioned object of the invention is achieved by a method in which, a light-emitting semiconductor structure is epitaxially grown onto the substrate layer of a transparent substrate produced in accordance with the method of producing an optically transparent substrate as presented above.
According to another mode of the invention, a light-emitting diode chip with an active light-emitting diode structure based on (AlGa)InP or AlGaAs is produced. A GaAs substrate is used as the lattice-matched substrate. An epitaxially grown (AlGa)InP layer or AlGaAs layer is used as the substrate layer. A GaP substrate is used as the optically transparent layer, and the active light-emitting diode structure is grown onto the substrate layer after the step of removing the lattice-matched substrate.
The method according to the invention is in particular preferably used for the production of light-emitting semiconductor chips with LED structures based on (AlGa)InP or AlGaAs, for which the problem explained above is especially relevant. It may, however, also be used for the production of other radiation-emitting semiconductor chips, in the case of which the substrate properties are important for the functional properties of the chip.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing an optically transparent substrate and a method for producing a light-emitting semiconductor chip, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.